1. Technical Field
The present disclosure relates to a device for detecting and measuring the distance to an object, based on a propagation time of a photon beam emitted in the form of pulses and reflected on the object. The present disclosure applies in particular to touch screens such as those used in mobile phones, and to the detection of an object near such a screen.
2. Description of the Related Art
It is known to use photodiodes as a distance detection and measurement element, using an avalanche phenomenon which may occur in the PN junctions of photodiodes. An avalanche phenomenon may occur in a diode PN junction when the diode is reverse biased near the junction breakdown voltage. This phenomenon can be used in two ways in an avalanche photodiode. If the avalanche photodiode is reverse biased just below the breakdown voltage, the photodiode then generates an electric current proportional to the intensity of the photon flux received by the photodiode, with a gain of some hundreds with a semiconductor such as silicon.
To detect low intensities of photon flux, it is known to use photodiodes which may be reverse biased above the breakdown voltage. Such photodiodes are called Single Photon Avalanche Diode (SPAD) or diodes operating in “Geiger” mode. Each time such a photodiode receives a photon, an avalanche phenomenon occurs in the photodiode PN junction, generating an intense current. To avoid destructing the photodiode by this intense current, the photodiode is connected to a quenching circuit allowing the avalanche process to be stopped some nanoseconds after appearing.
To perform a distance measurement, it is known to light a detection area with a pulsed light source such as a pulsed laser source, and to detect photons reflected by an object present in the detection area using a set of SPAD photodiodes. The distance from the object in the detection area is evaluated based on the propagation time or Time Of Flight (TOF) between the transmit time of a light pulse and the receipt time of a pulse across a photodiode, resulting from the photodiode avalanche triggering. The measure accuracy depends in particular on the duration of the light pulses emitted by the source; the shorter these pulses, the more accurate the measurement.
In an integrated circuit of complementary metal oxide semiconductor (CMOS) type, powered by a voltage around 3 to 5 V, the reverse bias of SPAD photodiodes to a voltage greater than the breakdown voltage is achieved with a bias voltage around 14 V. Such a voltage is produced by a high voltage generation circuit, for example based on a charge pump allowing the supply voltage to be increased. The high voltage generation circuit is sized taking into account the most unfavorable case where all the photodiodes of the detection circuit avalanche trigger at each pulse of a series of photon pulses emitted by the source. In addition, the detection circuit is all the more efficient, and in particular sensitive to detect an object, even a little reflecting one, since it comprises a great number of photodiodes. A SPAD photodiode may consume on average several microamperes if it avalanche triggers after each pulse of a series of photon pulses. This current is to be multiplied by the number of SPAD photodiodes in the circuit. The result is that the more photodiodes in the circuit, the greater the surface occupied by the high voltage generator, and the greater its electrical consumption, even in the absence of load.
The high voltage generation circuit therefore constitutes an element limiting the number of SPAD photodiodes of a detection circuit, in particular in the applications of the integrated circuit to a portable object such as a mobile phone, which preferably have an electrical consumption and dimensions as low as possible.